There are a vast number of complex metal oxides which contain two or more metal cations. The complex metal oxides are present as a homogeneous phase having a generally well defined crystalline structure wherein the metal atoms and oxygen atoms are organized into a compact network according to the requirements of the neutralization of the charges of the metal cations and oxygen anions. The complex metal oxides occur in numerous basic crystalline structural types, the names of which are often derived from the principle compound shown to have that type of crystal structure. For example, spinels which have the structure of the mineral spinel, MgAl.sub.2 O.sub.4, have the general formula AB.sub.2 O.sub.4, and perovskites which have the structure of the mineral perovskite, CaTiO.sub.3, have the general formula ABO.sub.3, wherein A and B are metals and where A and B occupy respective sites in the crystal lattice. Often, a portion of either or both the A and B sites are replaced with other metal cations.
The complex metal oxides are of growing importance to industry. Spinels and perovskites, for example, are often piezoelectric and ferromagnetic materials and as well are known to have important magnetic, optical and electrical properties. Very recently, it has been discovered that certain complex metal oxides are superconducting at anomalously high critical temperatures (T.sub.c). The crystal structure of one group of these newly discovered superconductive complex metal oxides is believed to be one similar to the crystal structure of K.sub.2 NiF.sub.4 and has a general complex metal oxide formula of A.sub.2 BO.sub.4 wherein A is lanthanum and a small portion of lanthanum sites are replaced with Ba or Sr. B is copper. The crystal structure is a layered one containing a CuO.sub.6 octahedral coordination at the corners like that of a perovskite separated by La,SrO layers. Another complex metal oxide with the formula YBa.sub.2 Cu.sub.3 O.sub.7 also contains perovskite structure and has been found to be superconducting at unusually high temperatures. The high T.sub.7 which has been found in these metal oxide superconductors holds much promise that such superconductive materials will be in common use.
Complex metal oxides are generally prepared by coprecipitating the metal salts or by weighing the metal oxides which form the complex metal oxide product to provide the necessary stoichiometric amounts of each metal and repeatedly grinding and firing the precursor mixture. While the grinding procedure has yielded the desired complex metal oxide, the method is time consuming, inasmuch as sufficient grinding is necessary to provide a homogeneous mixture of the respective metal cations on an atomic scale. Further, it is often difficult to obtain complex metal oxides in which the respective metals are present in the proper weight ratio to provide the desired crystalline structure such as a spinel, perovskite, etc. To prepare complex metal oxides from a mixture of separate metal salts or oxides, the amount of the respective metal salts or oxides, as well as the mixing and calcining conditions all have to be considered to obtain the desired structure. Typically, crystals of the complex metal oxides obtained by the prior art techniques contain several metallic phases, including one or more metal oxide phases of each respective metal, the desired homogeneous complex metal oxide phase, and other complex metal oxide phases. Not surprisingly, the presence of non-homogeneous metal oxide phases has been reported in the preparation of the superconducting complex metal oxides. For example, it has been reported that preparations using the traditional method of coprecipitation have not proved satisfactory for the superconducting YBa.sub.2 Cu.sub.3 O.sub.7 composition. The presence of a non-superconducting phase, the result of inhomogeneous stoichiometry, is a recurring problem in methods that rely on mechanical mixing and, apparently, even the coprecipitation procedure that generally gives good dispersion can result in impure material. Accordingly, a method that disperses the metals on an atomic scale in the desired stoichiometry is needed to overcome this problem.
A technique for forming perovskites, in particular, the preparation of rare earth ferrites and analogous cobalt compounds has been reported by Bell Telephone Laboratories, Gallagher, P.K., Materials Res. Bull., 3, 1968, pp. 225-32, in which rare earth ferrocyanide or cobalticyanide compounds, e.g., LaFe(CN).sub.6 XH.sub.2 O were precipitated from aqueous solution. For example, lanthanum chloride was dissolved in water and precipitating agents such as ammonium ferrocyanide and potassium cobalticyanide were added. Upon calcination of the precipitate in air at 1,000.degree. C., pure perovskites of LaFeO.sub.3 and LaCoO.sub.3 were formed with no evidence of La.sub.2 O.sub.3, Fe.sub.2 O.sub.3 or nonperovskite mixed metallic compounds. The advantages of the technique are the nearly perfect control of the rare earth to transition metal ratio at unity, complete homogeneity of the metallic compounds, good purity and ease of preparation because the normally required repetitive grindings and heating are eliminated, and finally a means of control over the particle size of the desired material by means of the choice of conditions for calcination.
An analogous system has been reported for forming BaTiO.sub.3. The method involves reacting barium chloride with potassium titanyl oxylate in which calcination of the precipitated barium titanyl oxylate results in the barium titinate (BaTiO.sub.3), Journal of the American Ceramic Society, Vol. 46, No. 8, Aug. 21, 1963, p. 359, and Journal of Research of the National Bureau of Standards, Vol. 56, No. 5, May 1956, p. 289. None of these articles address the formation of complex metal oxides other than perovskites in which the metals are present in a 1:1 ratio.
In recent years the formation and use of inorganic fibers, in particular, metal oxide fibers, have received increasing attention. Such interest may be traced at least in part to new demands of industry for fibers capable of withstanding elevated temperatures without deleterious effects. The growing aerospace industry, for example, provides many applications for light and strong heat resistant fibrous materials.
The formation of inorganic metal-containing fibers has required elaborate and time-consuming procedures inasmuch as such compounds possess high melting points which render melt-spinning either impossible or extremely difficult. Accordingly, procedures for forming inorganic fibers usually involve the addition of metal or metallic compounds to polymeric spinning solutions or to already spun polymeric fibers and subsequent thermal conversion to metal carbides, metal nitrides, metal oxides, etc. wherein the polymeric material is either used as a carbon source for forming carbides or is simply used as a carrier for the metallic components and consumed during thermal treatment.
In the formation of metal oxide fibers, the metals are often present in the form of inorganic or organic salts which are added to the spinning polymer solution or such salts are imbibed in preformed polymeric fibers. Upon calcination in air, the metal salts are converted to the respective metal oxides. Examples of such procedures are disclosed in U.S. Pat. Nos. 3,846,527; 4,010,233, 4,485,085 and 4,541,973. The two latter patents are directed to the formation of ferrimagnetic spinel fibers. U.S. Pat. No. 4,541,973 adds the powdered spinel into a spinning solution. In the methods described in the other mentioned patents, the metals are added as metal salts containing the metal species in the cationic form. Even in the formation of complex metal oxide fibers, the respective metal compounds are added separately to the spinning solution or imbibed into the preformed fibers in the form of various metallic salts in which each metal is a cationic species. Exact control over the proper weighing of the various metal salts are thus needed to form the desired metal oxide product. As stated above, it is often difficult to provide the stoichiometric amount of each metal when added in the form of metal salts so as to yield the desired mixed metallic oxide. It is difficult as well to provide the proper mixing of the metal salts and calcining conditions. Thus, oxide phases other than the desired homogeneous complex metal oxide phase are usually formed.
In view of the growing applications for complex metal oxides it would be worthwhile to provide complex metal oxide fibers and adapt such fibers for a wide range of prospective applications involving optics, electronics, as well as piezoelectric, magnetic, and superconductive applications. However, there remains a need for a new and improved process for the production of complex metal oxide fibers where the composition of the complex metal oxide can be more readily controlled to yield the proper ratio of the metal cations and form increasingly purer homogeneous complex metal oxide phases.
Accordingly, it is an object of this invention to provide an improved process for the production of complex metal oxide fibers.
It is another object of this invention to provide a process for the production of complex metal oxide fibers of improved purity.
Still another object is to provide a process for the production of complex metal oxide compositions of improved purity.
Yet another object is to provide an improved process for producing superconductive complex metal oxide compositions.
Other objects and advantages of the present invention shall become apparent from the accompanying description of the invention and appended claims.